A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, one or more parameters of the patterned substrate are typically measured, for example the overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in a lithographic process, including the use of a scanning electron microscope and various specialized tools. One form of specialized inspection tool is a scatterometer SM1 in which a beam of radiation is directed onto a target on the surface of the substrate and one or more properties of the scattered or reflected beam are measured. By comparing one or more properties of the beam before and after it has been reflected or scattered by the substrate, one or more properties of the substrate may be determined. This may be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with a known substrate property. Two main types of scatterometer SM1 are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angularly resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle. An ellipsometer also measures polarization state.
A scatterometer is typically used to detect a fault in an exposed (i.e. patterned) layer on a substrate and thereby potentially one or more corresponding faults in the lithographic apparatus used to form the patterned layer on the substrate. This sort of fault detection and classification (FDC) is currently carried out using metrology performance metrics such as measuring the overlay of overlapping gratings in subsequent patterned layers, and critical dimension (CD) or sidewall angle of a structure formed on the substrate. FDC carried out in this way is discussed in SEMI standard E133-0705. However, using a scatterometer to determine the exact shape of a structure on a layer is typically time consuming and processing power intensive.
An alternative to metrology performance metrics is the use of process tool data. The way this is done is that sensor and/or actuator data of a process tool is monitored. This data (or variation in data from one run to the next) is used as an indicator for a possible fault in the respective process or in previous processes. In most known implementations of FDC on metrology metrics, the performance metric (for example the overlay) is measured on a few sites of a few substrates per “lot” of substrates on dedicated metrology targets such as overlay targets that are normally present in the scribe lane.
The following is a description of fault measuring known in the art. A scatterometer such as that described above can be used to measure the overlay—that is the position of an exposed target with respect to one or more previous targets on the same site of the substrate in a previous layer. A scatterometer is alternatively used to turn raw spectral data into grating profile parameters such as CD (i.e. width of generally bar-like structures) and sidewall angle.
However, a scatterometer such as discussed above is not very practical in trying simply to determine whether a system is running well, as scatterometry requires a lot of processing power in order to obtain precise measurements of exactly what is happening at the substrate surface level.
In order to reduce the computation time, a previous method has simply reduced the number of sites per substrate used in the calculations. A problem with using only a limited number of sites per lot of substrates is that only a limited number of data points are available for each FDC performance metric. As few data points are used as is still feasible to keep computation time to a minimum. In other words, it has been desirable to reduce the number of data points, potentially sacrificing accuracy, in order to reduce the computation time.
An alternative is the use of a scanning electron microscope (SEM) to measure the width, predominantly, of the structures of a printed image with great precision. However, this is also a time-consuming method of determining precise measurements of the substrate (or product layer) surface as the substrate containing the product layer, etc., has to be removed from a lithographic cell and placed in the SEM at each time when measurement is required. If the aim is to track changes that might be an error in an exposure system, using an SEM is a time-inefficient way to do this.